The present invention is directed to an improved process for increasing the Si/Al ratio of a crystalline aluminosilicate zeolite. In particular, a process is disclosed for the selective removal of amorphous non-framework alumina from a zeolite. The present invention is also directed to the zeolites prepared by this process.
Crystalline aluminosilicate zeolites are essential ingredients in most catalysts for catalytic cracking of petroleum gas oils into blendstocks for premium transportation fuels. Preferred zeolites for cracking applications exhibit low sodium content (.ltoreq.0.3 wt. %) and framework (Si/Al) atomic ratios in the range of about 8-30. Zeolites with these ratios promote high activity and selectivity for gas oil conversion into gasoline range products with low selectivity for undesirable hydrogen transfer and coking reactions. Unfortunately, zeolites with this composition are not readily available for incorporation into fresh cracking catalysts. Rather, fresh cracking catalysts are frequently prepared using rare-earth-Y (Si/Al.sup..about. 2.5), ultrastable-Y (Si/Al.sup..about. 5), or LZ-210 (Si/Al.sup..about. 3-8) type zeolites with low (Si/Al) ratios. During the cyclic cracking operation, the framework (Si/Al) ratio gradually increases due to periodic steaming in the catalyst regenerator. Hydrothermal dealumination in this manner produces a "detrital alumina" (amorphous non-framework) phase on the surface of the zeolite particles. This detrital alumina phase is undesirable because it tends to promote coking reactions and block access to the zeolite internal pore structure.
To minimize the impact of detrital alumina on catalyst performance, it would be very desirable to directly incorporate high (Si/Al) ratio zeolites in the catalyst preparation. The present invention herein described is directed to a method of preparing zeolites with high framework (Si/Al) ratios from corresponding zeolites with low (Si/Al) ratios such as USY or LZ-210. This method results in zeolites largely free of detrital alumina and retaining a high level of zeolite crystallinity. With appropriate choice of conditions, the framework (Si/Al) ratio can be tailored over a broad range of composition, e.g., (Si/Al).sup..about. 8-50. High (Si/Al) ratio zeolites prepared in this manner are particularly effective as solid acid components in fluid cat cracking catalysts.
The present method of preparing high (Si/Al) ratio zeolites is based on a two-step procedure involving (1) hydrothermal dealumination, and (2) selective extraction of detrital alumina produced during the hydrothermal dealumination. Advantageously, the (Si/Al) ratio can be largely controlled by the initial steaming conditions.
The concept of steaming and acid extraction to produce high silica zeolites is not new. In previous work, extraction was carried out with common mineral acids such as HCl, H.sub.3 PO.sub.4, HNO.sub.3, and the like. A major drawback of this approach is based on the fact that mineral acid extraction is highly non-selective for detrital alumina. Indeed, mineral acids extract both framework and non-framework aluminum producing zeolites with low aluminum content, irregular aluminum distributions, and reduced crystallinity.
The prior art teaches possible methods of increasing silica to alumina mole ratios of crystalline aluminosilicate zeolites beyond either synthetic upper limits or that of the naturally occurring material. Several patents teach extraction of an aluminum from zeolite, however, not involving steaming prior to extraction. Pickert U.S. Pat. No. 3,640,621 discloses a process for producing high (Si/Al) faujasite by direct extraction of calcined zeolite with a non-acidic chelating agent such as acetylacetone. Young U.S. Pat. No. 3,691,099 teaches a process for improving the properties of oxides by direct extraction with mineral or organic acid salt solutions such that the pH is not so low as to destroy the crystalline structure of solid oxide. Chang et al U.S. Pat. No. 4,273,753 teaches a dealumination method based on direct reaction of a zeolite with an inorganic halide or oxyhalide at a high enough temperature to volatilize the resulting aluminum halide. U.S. Pat. No. 4,297,355 teaches modifying a zeolite by fluorine treatment in order to alter the zeolite's adsorption properties. Acid treatments of clays are disclosed in Maher et al U.S. Pat. No. 3,393,045 and Hildebrandt U.S. Pat. No. 3,492,088.
It is known to steam a zeolite in some cases prior to an extraction step. U.S. Pat. No. 3,493,519 to Kerr et al discloses producing a high (Si/Al) faujasite by steaming followed by extraction with a chelating agent at pH 7-9. The preferred chelating agent is the ammonium salt of ethylenediaminetetraacetic acid (EDTA). Eberly U.S. Pat. No. 3,506,400 teaches preparing a high silica zeolite by steaming and acid extraction of the zeolite. Eberly teaches that a chelating agent may be employed in place of an acid to remove non-framework amorphous alumina. Exemplary extracting agents in the Eberly patent are EDTA, sulfonic acid resin, and 0.1N HCl. The data in Tables III and VI of the Eberly patent show that framework aluminum is also leached during extraction under the conditions applied.
Another approach to zeolite modification is to protect sites in a zeolite by temporarily loading or covering with a foreign material during extraction. Dewing et al U.S. Pat. No. 4,533,533 teaches dealumination of framework sites near the external zeolite surface by protecting the interior with preadsorbed coke or other foreign material. Kokotailo et al U.S. Pat. No. 4,415,544 teaches surface modification of certain ZSM-type zeolites by contacting wax loaded materials with HF solutions. The latter two patents do not teach steaming prior to extraction.